Arsenic trioxide (AS2O3) is a heavy metal derivative that has potent antileukemic properties in vitro and in vivo. This agent is used in the treatment of patients with acute promyelocytic leukemia (APL), but the precise mechanisms by which it induces its antileukemic effects are not known. We have identified a novel signaling pathway activated by arsenic trioxide in APL cells, involving the p38 Map kinase. Our data suggest that activation of this signaling cascade exhibits a negative regulatory role on the induction of apoptosis and cell differentiation of APL cells. The overall goal of this grant application is to understand the mechanisms by which p38 negatively regulates the induction of AS2O3-responses in APL cells. Specific aim A is to determine the mechanisms of activation of the p38 Map kinase by AS2O3 in APL cells. Studies will be performed to examine the roles of the small GTPases Rac1 and Cdc42, and the Pak1 kinase, and to identify the Map kinase kinase (Mkk) that directly phosphorylates and activates p38. Specific aim B is to identify the downstream effector mechanisms by which the p38 Map kinase controls AS2O3-dependent apoptosis. Studies are proposed to examine the patterns of activation of different p38-isoforms in APL cells and to determine their roles in the generation of AS2O3-responses. Experiments will be also performed to dissect the contributions of different downstream effectors of the p38 pathway in the regulation of such responses. Specific aim C is to examine the activation of p38 in primary leukemic blasts from APL patients or patients with other subtypes of acute myeloid leukemia (AML), and determine whether such activation correlates with sensitivity or resistance to the effects of arsenic trioxide. Altogether, these studies should advance our overall understanding of the mechanisms by which AS2O3 generates its effects on malignant cells. They may also provide the basis for future clinical-translational efforts of combinations of AS2O3 and p38 inhibitors for the treatment of APL, and possibly other forms of AML.